Alternative path cooling of a high temperature fuel cell

ABSTRACT

Systems and methods provide for the thermal management of a high temperature fuel cell. According to embodiments described herein, a non-reactant coolant is routed into a fuel cell from a compressor or a ram air source. The non-reactant coolant absorbs waste heat from the electrochemical reaction within the fuel cell. The heated coolant is discharged from the fuel cell and is vented to the surrounding environment or directed through a turbine. The energy recouped from the heated coolant by the turbine may be used to drive the compressor or a generator to create additional electricity and increase the efficiency of the fuel cell system. A portion of the heated coolant may be recycled into the non-reactant coolant entering the fuel cell to prevent thermal shock of the fuel cell.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication No. 61/046,052, filed on Apr. 18, 2008, and entitled“Alternative Path Cooling of a High Temperature Fuel Cell,” which isexpressly incorporated herein by reference in its entirety.

BACKGROUND

A fuel cell operates by creating an electrochemical reaction betweenincoming fuel and oxidizer streams to create electricity. Many fuelcells, such as solid oxide fuel cells (SOFC), operate at hightemperatures. Waste heat created by the electrochemical reaction withina fuel cell must be removed to control the temperature of the fuel cellto prevent failure of the cell. A typical thermal management systemincludes circulating excess reactant, beyond what is needed for theelectrochemical reaction, through the fuel cell to absorb heat. However,in certain applications, such as an airborne application in which a fuelcell is utilized on an aircraft, weight is a primary consideration.Storing the excess reactant required to maintain the temperature of thefuel cell can be weight prohibitive.

Gases exiting the fuel cell can be recirculated back to an incomingreactant stream for cooling purposes. However, doing so requires anadditional cooling subsystem to cool the recycled flow due to the heatabsorbed from the fuel cell. The additional cooling subsystem results ina more complex control system, additional vehicle thermal load, and anincreased weight of the overall system. Another conventional thermalmanagement system includes a separate closed system cooling loop forcirculating a stored coolant through the fuel cell and through a coolingsubsystem. Similar to the other thermal management systems describedabove, a separate closed system cooling loop adds additional weight andadds complexity with additional pumps, coolant, lines, and powerconsumption requirements.

It is with respect to these considerations and others that thedisclosure made herein is presented.

SUMMARY

It should be appreciated that this Summary is provided to introduce aselection of concepts in a simplified form that are further describedbelow in the Detailed Description. This Summary is not intended to beused to limit the scope of the claimed subject matter.

Systems and methods described herein provide for the cooling of a fuelcell using an ambient coolant, such as air or water depending on theapplication. According to one aspect of the disclosure provided herein,a coolant that is separate from the reactants is provided to the fuelcell. The coolant flows through the fuel cell to absorb heat beforebeing discharged and directed away from the fuel cell. According tovarious embodiments, the incoming ambient coolant may be directedthrough the fuel cell using ram pressure from the movement of thevehicle associated with the fuel cell through the environment, or usinga flow control device such as a compressor or pump. Embodimentsadditionally provide for directing the heated coolant from the fuel cellto a turbine, which in turn may be used to drive a generator forcreating additional electricity or to drive the compressor. The heatedcoolant may also be recirculated into the incoming ambient coolant toincrease the temperature of the ambient coolant before it enters thefuel cell to prevent damaging the fuel cell through thermal shock.

The features, functions, and advantages that have been discussed can beachieved independently in various embodiments of the present inventionor may be combined in yet other embodiments, further details of whichcan be seen with reference to the following description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a fuel cell cooling system accordingto various embodiments presented herein;

FIG. 2 is a block diagram showing a fuel cell cooling system thatutilizes a turbine-driven compressor according to various embodimentspresented herein;

FIG. 3 is a block diagram showing a fuel cell cooling system thatutilizes recirculated coolant to increase the temperature of the ambientcoolant entering the fuel cell according to various embodimentspresented herein;

FIG. 4 is a schematic diagram showing the flow of coolant through a fuelcell cooling system to illustrate heat transfers at various stageswithin the system according to various embodiments presented herein; and

FIG. 5 is a flow diagram illustrating a method for controlling thetemperature of a high temperature fuel cell according to variousembodiments presented herein.

DETAILED DESCRIPTION

The following detailed description is directed to systems and methodsfor controlling the temperature of a fuel cell. As discussed brieflyabove, fuel cell systems include two input flows, a fuel and anoxidizer. Typical thermal management systems utilize excess reactantflow and/or separate closed system cooling loops to control thetemperature of the corresponding fuel cell. These systems are not alwaysoptimal when the fuel cell operates in a vehicle or platform havingstringent weight, space, and power constraints.

Throughout this disclosure, for illustrative purposes, the variousembodiments will be described with respect to the operation of a hightemperature fuel cell, such as a SOFC, used to create electrical powerfor an aircraft or aircraft subsystem. However, it should be understoodthat the disclosure provided herein is equally applicable to any type offuel cell used in any application in which an ambient flow of coolant isreadily available. As an example of “coolant” as disclosed herein,aircraft and vehicles propel themselves through the surrounding air,creating an ambient airflow that may be used as a coolant according tothe various embodiments described herein. Similarly, vessels such asships and submarines propel themselves through the surrounding water,creating an ambient water flow that may be used as the coolant describedbelow with respect to various embodiments. Accordingly, the flow of the“ambient coolant” and “coolant” described herein applies to the flow ofany fluid, and according to various embodiments, to the fluid within theenvironment surrounding the platform containing the fuel cell to becooled. The flow of the “ambient coolant” also applies towardsstationary systems where the coolant flow is driven into the system.

Utilizing the concepts and technologies described herein, a hightemperature fuel cell system may be operated in a manner that allows forthermal control of the fuel cell stack, utilizing a coolant stream thatis separate from the reactant streams flowing into the fuel cell. Onceheated by an exchange of heat within the fuel cell, the heated coolantmay be utilized to create additional electricity and/or drive furthersystem components as described below. Because the separate coolantstream is input to the fuel cell system from the ambient environmentrather than storage, and because the output coolant containing the wasteheat from the fuel cell may be utilized to drive system components andcreate additional power, the embodiments disclosed herein provide for anefficient, weight-effective thermal management system.

In the following detailed description, references are made to theaccompanying drawings that form a part hereof, and which are shown byway of illustration, specific embodiments, or examples. Referring now tothe drawings, in which like numerals represent like elements through theseveral figures, thermal management of a high temperature fuel cellcooling system will be described. FIG. 1 shows a high temperature fuelcell cooling system 100 according to one embodiment described herein.The high temperature fuel cell cooling system 100 includes a fuel cell102 that is operative to create products 106, such as water andelectricity, from an electrochemical reaction of the reactants 104,which include oxygen and a fuel.

A non-reactant coolant 108 is routed to the fuel cell 102. According toone embodiment, the non-reactant coolant 108 includes ram air capturedby the high temperature fuel cell cooling system 100 from ambient airrushing past the vehicle containing the high temperature fuel cellcooling system 100. A coolant supply mechanism for supplying the ram airto the fuel cell 102 may include ducting and any other components thatcapture the ambient airflow and transport it into and through the fuelcell 102. An example of an alternative coolant supply mechanism will bedescribed below with respect to FIG. 2.

Within the fuel cell 102, the ambient air can be routed throughout thefuel cell via ducts, conduit, apertures, or other channels to absorbwaste heat from the electrochemical reaction inside. In this manner, thefuel cell 102, or portions of the fuel cell that thermally contact thenon-reactant coolant 108, operates as a heat exchanger, transferringheat from the higher temperature fuel cell to the lower temperaturenon-reactant coolant 108. The heated coolant 110 containing thenon-reactant coolant 108 saturated with waste heat is discharged fromthe fuel cell 102.

The high temperature fuel cell cooling system 100 may include a heatdisposal mechanism that receives the heated coolant 110 from the fuelcell 102 and disposes or otherwise utilizes it. The heated coolant 110may be utilized in any number and combination of ways according tovarious embodiments described herein. First, the heat disposal mechanismmay simply include ducting and components for venting the heated coolant110 to the ambient environment. For example, ram air that is forcedthrough the fuel cell 102 may be discharged to the atmosphere.

An alternative heat disposal mechanism may include a turbine 112, asshown in FIG. 1. The turbine 112 may be coupled to a generator 114. Inthis embodiment, the heated coolant 110 drives the turbine 112, which inturn drives the generator 114 to create electricity. The generator 114can provide power to an aircraft system in addition to the power createdby the electrochemical reaction within the fuel cell 102. Heated coolant110 leaving the turbine 112 may be vented to the ambient environment.The heat disposal mechanism may additionally include a recirculationdevice 302, which will be described in detail below with respect to FIG.3.

FIG. 2 shows an alternative high temperature fuel cell cooling system200 that utilizes an alternative coolant supply mechanism. Specifically,the alternative coolant supply mechanism includes a compressor 202 forsupplying the non-reactant coolant 108 to the fuel cell 102. Accordingto this embodiment, ram air is not utilized to cool the fuel cell 102.Rather, low pressure, non-reactant ambient airflow is routed to thecompressor 202, which provides some pressurization of the non-reactantcoolant 108 to supply it to the fuel cell 102. As an example, in aregenerative high altitude aircraft power system, oxygen and hydrogenare both stored under pressure. However, since the cooling air stream isnot reactive, it would not be necessary to highly pressurize the stream.As a result, the compressor 202 may utilize a single stage systemwithout any interstage cooling.

Further according to this embodiment, the turbine 112 of the heatdisposal mechanism described above for managing the heated coolant 110from the fuel cell 102 is used to mechanically drive the compressor 202.The compressor may also be driven by other means such as an alternativeelectrical source or from the generator (114). It should be understoodthat the turbine 112 and the generator 114 may be utilized tomechanically or electrically drive any number and type of desiredplatform components within the scope of this disclosure, provided thatthe characteristics of the heated coolant 110 allow for the desiredturbine 112 and generator 114 output.

Looking at FIG. 3, yet another alternative high temperature fuel cellcooling system 300 includes a recirculation device 302 to route aportion of the heated coolant 110 back into the non-reactant coolant 108flowing into the fuel cell 102. The recirculation device 302 may includea fan or ejector that is operative to supply the heated coolant 110 tothe non-reactant coolant 108 stream entering the fuel cell. Because ofthe significant temperature differential that may be present between theambient coolant exiting the compressor 202 and the fuel cell 102, theremay be a high potential for damage to the fuel cell 102 due to thermalshock that would occur from utilizing a non-reactant coolant 108 that issignificantly cooler than the fuel cell 102 reaction. As a result,embodiments described herein provide for the heating of the non-reactantcoolant 108 stream to a temperature higher than the temperature of thenon-reactant coolant 108 exiting the compressor 202, but lower than thatof the fuel cell 102.

According to various embodiments, heating the non-reactant coolant 108with the heated coolant 110 upstream from the fuel cell 102 may occurthrough an actual mixing of the two flows or via thermal contact betweenthe two flows without commingling the non-reactant coolant 108 and theheated coolant 110. To transfer heat from the heated coolant 110 to thenon-reactant coolant 108 without commingling the two flows, arecuperator, or heat exchanger, may be used. An implementation utilizinga recuperator will be described with respect to FIG. 4.

FIG. 4 illustrates the path of coolant flow through a high temperaturefuel cell cooling system 400. Various heat transfers at different stageswithin the system will now be discussed using illustrative temperaturevalues. It should be understood that the temperature values describedare for illustration purposes only. The actual temperature differentialsbetween the various stages of the high temperature fuel cell coolingsystem 400 will depend on any number of factors, including but notlimited to the operating characteristics of the fuel cell 102, the heatcapacity of the non-reactant coolant 108, the flow rates of thenon-reactant coolant 108 throughout the system, the operationalspecifications of the compressor 202 and the turbine 112, and thecharacteristics of the recuperator 402, among others.

At stage 1, the non-reactant coolant 108 enters the compressor 202 at−51 C as an ambient airflow from outside of an aircraft at altitude. Thenon-reactant coolant 108 heats as it is pressurized by the compressor,exiting the compressor 202 at 84 C at stage 2. From stage 2, thenon-reactant coolant 108 enters the recuperator 402. As described above,the recuperator 402 is a heat exchanger that transfers heat from heatedcoolant 110 from the fuel cell 102 to the non-reactant coolant 108entering the fuel cell 102 in an effort to prevent thermal shock fromdamaging the fuel cell 102 as a result of an excessive temperaturedifferential between the non-reactant coolant 108 entering the fuel cell102 and the heat within the fuel cell 102. After heating thenon-reactant coolant 108 within the recuperator 402, the non-reactantcoolant 108 exits the recuperator 402 and enters the fuel cell 102 at625 C at stage 3.

The non-reactant coolant 108 absorbs further heat within the fuel cell102, becoming heated coolant 110. The heated coolant 110 exits the fuelcell 102 and re-enters the recuperator 402 at 800 C at stage 4. Theheated coolant 110 is used to heat the non-reactant coolant 108 withinthe recuperator 402. The heated coolant 110 exits the recuperator 402and enters the turbine 112 at 246 C at stage 5. The heated coolant isfurther cooled through the turbine 112, and exits the turbine 112 at 110C at stage 6.

Turning now to FIG. 5, an illustrative routine 500 for managing thetemperature of a high temperature fuel cell 102 will now be described indetail. It should be appreciated that more or fewer operations may beperformed than shown in the FIG. 5 and described herein. Moreover, theseoperations may also be performed in a different order than thosedescribed herein. The routine 500 begins at operation 502, where thenon-reactant coolant 108 is routed through the fuel cell 102. Asdescribed above, the non-reactant coolant 108 may be driven through thefuel cell 102 as ram air or using the compressor 202. At operation 504,heat from the fuel cell 102 is transferred to the lower temperaturenon-reactant coolant 108, creating the heated coolant 110. The heatedcoolant 110 is directed away from the fuel cell 102 at operation 506.

If the high temperature fuel cell cooling system 300 does not include aturbine 112 as part of a heat disposal mechanism at operation 508, thenthe routine 500 proceeds to operation 510, where the heated coolant 110is vented to the environment or partially recirculated if the hightemperature fuel cell cooling system includes a recirculatory system atoperation 522 as described below. However, if the high temperature fuelcell cooling system 100 includes a turbine 112, then the routine 500continues from operation 508 to operation 512. If a compressor 202 ispresent within the high temperature fuel cell cooling system 300 andutilized to provide the non-reactant coolant 108 to the fuel cell 102,then the routine 500 continues through operation 512 to operation 514,where the compressor is driven with the turbine 112. However, if thenon-reactant coolant 108 is provided to the fuel cell 102 directly asram air, then the routine 500 proceeds from operation 512 to operation516.

If a generator 114 is not present within the high temperature fuel cellcooling system 300 at operation 516, then the routine 500 proceeds tooperation 522 and continues as described below. However, if a generator114 is to be utilized within the high temperature fuel cell coolingsystem 300, then the routine continues to operation 518, where theturbine 112 is used to drive the generator 114 to create electricity. Atoperation 520, the electricity is routed to one or more systemsassociated with the platform on which the high temperature fuel cellcooling system 300 is being utilized. By utilizing the heated coolant110 to generate electricity in addition to the electricity generated bythe fuel cell 102 electrochemical reaction as described herein, theefficiency of the entire fuel cell system is increased.

If the high temperature fuel cell cooling system 300 does not include arecirculatory system for recycling heated coolant 110 back into thenon-reactant coolant 108 stream at operation 522, then the routine 500ends. However, if the high temperature fuel cell cooling system 300includes a recirculation device 302, then the routine 500 continues fromoperation 522 to operation 524, where a portion of the heated coolant110 is recirculated to the non-reactant coolant 108 stream entering thefuel cell 102. As described above, recirculation flow of the heatedcoolant 110 may be located earlier on in the flow system, such as beforethe turbine 112. The heated coolant 110 is used to increase thetemperature of the non-reactant coolant 108 stream so that thermal shockof the fuel cell 102 is prevented. The recuperator 402 may be used asdescribed above to enable the heat transfer from the heated coolant 110to the non-reactant coolant 108.

It should be clear from the various embodiments described above that thedisclosure provided herein provides a weight-efficient process formanaging the temperature of a high temperature fuel cell. By utilizing anon-reactant ambient air or water flow to cool the fuel cell 102,storing excess reactants 104 used for cooling purposes and providingseparate closed system cooling loops can be avoided. Moreover, the hightemperature fuel cell cooling system 300 described above providesflexibility to tailor the system according to the specific applicationparameters. For example, the non-reactant coolant 108 may be providedvia ram air or a compressor depending on the platform operationalparameters. Similarly, a turbine 112 may be added to the hightemperature fuel cell cooling system 300 to recapture some of the energywithin the heated coolant 110, which can then be used to drive thecompressor 202 and/or to generate additional electricity using thegenerator 114, increasing the overall efficiency of the fuel cellsystem.

The subject matter described above is provided by way of illustrationonly and should not be construed as limiting. Various modifications andchanges may be made to the subject matter described herein withoutfollowing the example embodiments and applications illustrated anddescribed, and without departing from the true spirit and scope of thepresent invention, which is set forth in the following claims.

What is claimed is:
 1. A system for thermal management of a fuel cell,the system comprising: a first reactant supply mechanism closed to anambient environment and configured to store and supply oxygen to thefuel cell for a fuel cell reaction; a second reactant supply mechanismclosed to the ambient environment and configured to store and supplyfuel to the fuel cell for the fuel cell reaction; and a coolant supplymechanism coupled to the ambient environment and isolated from the firstreactant supply mechanism and from the second reactant supply mechanismcomprising an inlet defining a channel through which a non-reactantambient airflow is captured from an ambient environment, the coolantsupply mechanism operative to provide an entire portion of thenon-reactant ambient airflow captured through the channel from theambient environment to the fuel cell; a heat exchanger within the fuelcell configured to receive the entire portion of the non-reactantambient airflow captured through the channel from the ambientenvironment from the coolant supply mechanism, route the entire portionof the non-reactant ambient airflow through a portion of the fuel cellto absorb heat from the fuel cell to create heated coolant, anddischarge the heated coolant from the fuel cell; and a heat disposalmechanism configured to receive the heated coolant discharged from theheat exchanger and direct the heated coolant away from the fuel cell. 2.The system of claim 1, wherein the non-reactant ambient airflowcomprises ram air, and wherein the heat exchanger comprises a conduitfor routing the ram air through the portion of the fuel cell to absorbheat from the fuel cell.
 3. The system of claim 1, wherein the coolantsupply mechanism comprises a compressor configured to pressurize thenon-reactant ambient airflow and to direct the non-reactant ambientairflow to the heat exchanger.
 4. The system of claim 1, wherein theheat disposal mechanism comprises a turbine configured to createmechanical energy from a flow of the heated coolant discharged from theheat exchanger.
 5. The system of claim 4, wherein the turbine is coupledto a generator, enabling production of electricity from the mechanicalenergy created from the flow of the heated coolant.
 6. The system ofclaim 4, wherein the coolant supply mechanism comprises a compressorconfigured to pressurize the non-reactant ambient airflow and to directthe non-reactant ambient airflow to the heat exchanger, and wherein theturbine is further configured to drive the compressor.
 7. The system ofclaim 1, wherein the heat disposal mechanism comprises an outletconfigured to vent the heated coolant to an ambient environment.
 8. Thesystem of claim 1, wherein the heat disposal mechanism comprises arecirculation device operative to combine a portion of the heatedcoolant with the non-reactant ambient airflow to increase a temperatureof the non-reactant ambient airflow and decrease a temperaturedifferential between the non-reactant ambient airflow and the heat fromthe fuel cell.
 9. The system of claim 1, wherein the fuel cell comprisesa solid oxide fuel cell that provides power to an aircraft system,wherein the non-reactant ambient airflow comprises ram air, wherein theheated coolant comprises heated ram air, and wherein the heat disposalmechanism comprises a turbine configured to create mechanical energyfrom a flow of the heated ram air discharged from the heat exchanger.